1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a facsimile machine, and a printer, and to an image forming device for use in the image forming apparatus.
2. Discussion of the Background
There is a need for an image forming apparatus using direct recording methods without producing images having uneven image density.
Japanese published unexamined application No. 63-136058 (JP-S63-136058-A), e.g., discloses a conventional image forming apparatus forming images by a direct recording method. The direct recording method is different from an indirect electrophotographic process of forming a latent image and attaching toner thereto, and forms a toner image by a direct process of selectively attaching a toner to a dot-formed area on which a latent image is not formed on a recording material.
FIG. 1 is a schematic view illustrating a main configuration in an image forming apparatus using a conventional direct recording method. In FIG. 1, a toner bearing roller 901 bearing a toner is located so as to extend its rotational axial line from side to side therein and is rotationally driven by an unillustrated driver. A circuit substrate 903 having plural through-holes 902 therein is located below the toner bearing roller 901 bearing a toner particle T on its surface. On the circumference of the through-hole 902, a ring-shaped flying control electrode 904 is formed as an adjacent electrode surrounding the hole.
Below the circuit substrate 903, a facing electrode 906 facing the toner bearing roller 901 through the circuit substrate 903 and a sheet of recording paper 907 fed by an unillustrated feeder in a direction perpendicular to the surface of the paper on the facing electrode 906 are located. The toner bearing roller 901, e.g., bears a negatively polarized toner T on the surface thereof while grounded. Among the plural through-holes 902, e.g., when a positively polarized recording on voltage is applied to the flying control electrode 904 surrounding images holes, i.e., the through-holes 902 located at an image area on the recording paper 907, an electrostatic force is applied to the toner particle T located at a position facing the flying control electrode 904 on the toner bearing roller 901. An aggregate of the toner particles T flies from the toner bearing roller 901 in the shape of a dot and enters the through-hole 902. The toner particles T continue to fly, being attracted by an electric field formed between the flying control electrode 904 and the facing electrode 906 having a potential higher than that of the flying control electrode 904, pass the through-hole 902 and adhere to the surface of the recording paper 907. The aggregate of the toner particles T forms a dot by the adherence.
In FIG. 1, only one combination of the through-hole 902 and the flying control electrode 904 (hereinafter “hole-electrode combination”) is shown, but there are plural combinations actually. For example, when a dot of 300 dpi is formed in whole area on a A4 size recording paper in its shorter side (210 mm) direction while fed along its longitudinal direction, straight-lined 2,482 dots in the main scanning direction (perpendicular to a feeding direction of the recording paper 907 as indicated by an arrow A in FIG. 2) form a line image. Therefore, 2,482 combinations of the through-hole 902 and the flying control electrode 904 are formed on the substrate 903. In terms of downsizing the apparatus they are preferably arranged in a line, but there are gaps between adjacent dots when so arranged. Therefore, 2,482 combinations are separately located in plural lines so that gaps among dots formed in a line are filled with other dots formed in other lines. For example, in FIG. 2, 2,482 combinations the through-hole 902 and the flying control electrode 904 are separately located in 8 lines (lines A to H).
The arrangement of the flying control electrode 904 will be explained in more detail. FIG. 50 is an amplified plain view illustrating a first embodiment of the arrangement of the flying control electrode 904. A direction indicated by an arrow B in FIG. 50 is a feeding direction (sub-scanning direction) of an unillustrated sheet of recording paper. A direction indicated by an arrow A in FIG. 50 is a direction perpendicular to a feeding direction (that is, a main scanning direction) of a recording paper. In FIG. 50, 8 electrodes from line A (first line) to line H (eighth line) are formed in the main scanning direction. The flying control electrode 904 located in an electrode line has a diameter of 300 μm. The through-hole 902 having a diameter of 150 μm is formed at the center of the flying control electrode 904. In each electrode line, such combinations of the flying control electrode 904 and the through-hole 902 are lined at a pitch of 4×β in the main scanning direction. In FIG. 50, β is a dot pitch of 169.3 μm to produce images having an image resolution of 150 dpi. Therefore, in each electrode line, the “hole-electrode combinations” are located at the same pitch as a dot pitch of 150/4=37.5 dpi. From line A (first line) to line D (fourth line), as shown in FIG. 50, locations of the “hole-electrode combinations” in the main scanning direction are shifted by “β” respectively. Therefore, four lines from line A (first line) to line D (fourth line) realize a dot pitch equivalent to an image resolution of 150 dpi in the main scanning direction. The “hole-electrode combination” in line E (fifth line) is located, as shown in FIG. 50, between the “hole-electrode combination” in line A (first line) and the “hole-electrode combination” in line B (second line) in the main scanning direction. Similarly, the “hole-electrode combination” in line F (sixth line) is located between the “hole-electrode combination” in line B (second line) and the “hole-electrode combination” in line C (third line), the “hole-electrode combination” in line G (seventh line) is located between the “hole-electrode combination” in line C (third line) and the “hole-electrode combination” in line D (fourth line), and the “hole-electrode combination” in line H (eighth line) is located between the “hole-electrode combination” in line D (fourth line) and the “hole-electrode combination” in line E (fifth line), respectively. Therefore, eight lines from line A (first line) to line H (eighth line) realize a dot pitch (α=84.6 μm) equivalent to an image resolution of 300 dpi in the main scanning direction. A Pitch “” of electrode lines in a sub-scanning direction (arrow B direction) is four times α (=338.7 μm). Total 8 “hole-electrode combinations” of one per line form a line image of 84.6×8=676.8 μm in the main scanning direction. The shorter side of A4 is 210 mm=210,000 μm, and 210,000/676.8×8=2,482 pieces of the “hole-electrode combinations” are formed to form a line image extending over the whole area of the shorter side direction.
FIG. 51 is an amplified plain view illustrating a second embodiment of the arrangement of the flying control electrode 904. In this second embodiment, the locations of the “hole-electrode combinations” are shifted by α=84.6 μm respectively in the main scanning direction and order of lines A, B, C, D, E, F, G and H. Such electrodes arrangement realizes an image resolution of 300 dpi as well as the first embodiment. In addition, 2,482 pieces of the “hole-electrode combinations” are formed to form a line image extending over the whole area of the shorter side direction.
The direct recording method needs individually turning on and off a record-on-voltage for plural flying control electrodes 904 using dedicated ICs, which are numerous. For example, 2,482 ICs are needed to form an image having an image resolution of 300 dpi. ICs are typically more expensive as they have higher voltage resistance, and it is important to keep a record-on-voltage as low as possible in the direct recording method. However, the record-on-voltage is at least 500 v or more to form an electric field overcoming adherence between the toner bearing roller 901 and the toner particles T (such as an image force, a van der Waals' force and a liquid cross-linking force). This has hindered efforts to decrease cost.
Image forming apparatuses using hopping development methods are conventionally known. The hopping development methods use toner hopping on the surface of a toner bearer and not toner adhering to a roller or a magnetic carrier. For example, JP-2007-133387-A discloses an image forming apparatus having a cylindrical toner bearer having plural hopping electrodes located at a predetermined pitch in a circumferential direction of the toner bearer. The same repeated pulse voltage of A phase is applied to the hopping electrodes in even-numbered lines, while the same repeated pulse voltage of B phase is applied to the hopping electrodes in odd-numbered lines. An alternating electric field is formed between the two hopping electrodes next to each other to cause the toner to hop to and fro between an A phase electrode and a B phase electrode. The toner bearer rotates to feed the hopping toner T to a developing area facing a latent image bearer to develop the latent image.
The hopping development method includes a method of transferring a toner to a developing area without a rotational surface movement of the toner bearer. JP-2002-341656-A discloses an image forming apparatus transferring a toner to a developing area as follows. Namely, in the image forming apparatus, plural electrode combinations formed of three electrodes of an A phase electrode, a B phase electrode and a C phase electrode, lined up in this order, are arranged on a toner bearer in a line. On the surface of the toner bearer, toner is repeatedly made to hop from the A phase electrode to the B phase electrode, the B phase electrode to the C phase electrode, and the C phase electrode to the A phase electrode, in this order. This hopping transfers the toner from one end of the toner bearer to a developing area at the other end.
In any hopping development method, toner is caused to hop on the surface of the toner bearer to eliminate adherence between the toner and the toner bearer. JP-S59-181370-A discloses an image forming apparatus applying this principle to a direct recording method. The image forming apparatus uses a method of recording a dot by passing toner hopping on the surface of the toner bearer through an image hole in a circuit board (hereinafter referred to as a hopping direct recording method). The method largely reduces the record-on voltage. This is because the hopping eliminates adherence between the toner and the toner bearer and an electric field need not be stronger than the adherence to pass toner through the image hole in the circuit board.
However, the present inventors have found that the hopping direct recording method is likely to produce images having uneven image density because an amount of toner entering each hole on the circuit board varies. This will be explained in detail.
FIG. 52 is an amplified plain view illustrating the “hole-electrode combination” of the first embodiment in FIG. 50 with a hopping electrode 911 of a toner bearing sleeve. As shown in FIG. 52, the hopping electrode 911 for hopping a toner on the surface of the toner bearing sleeve has the shape of a reed extending in the main scanning direction (arrow A). The reed-shaped hopping electrodes 911 are arranged in multiple parallel lines at a pitch of δ=250 μm in a sub-scanning direction (arrow B). The hopping electrode 911 has a width of 100 μm. The gap G between the hopping electrodes 911 is 150 μm. When a pulse voltage is applied to the hopping electrode 911, toner moves reciprocally between the hopping electrodes 911 in the sub-scanning direction.
FIG. 3 is a schematic view illustrating a main configuration of an image forming apparatus using a hopping direct recording method. In FIG. 3, plural hopping electrodes 911 arranged in parallel lines at a predetermined pitch are circumferentially formed on a toner bearing sleeve 910 as a toner bearer. The hopping electrode 911 is the same as that in FIG. 52. On the surface of the toner bearing sleeve 910, toner hops between adjacent hopping electrodes 911 in a parabolic orbit having an apogee halfway between the hopping electrodes as shown in FIG. 3. In FIG. 3, the center of the through-hole 902 in the first line (left side) is located at almost the same position of the middle of the parabolic orbit of a hopping toner, and many toner particles are present close to the through-hole 902 in the first line. Therefore, comparatively many toner particles enter the through-hole 902 in the first line. By contrast, the center of the through-hole 902 in the second line (right side) is largely offset from the middle of the parabolic orbit of the hopping toner, and few toner particles are present close to the through-hole 902 in the second line. Therefore, comparatively few toner particles enter the through-hole 902 in the second line. This large difference in the amount of a toner entering the through-hole 902 causes uneven image density.
For this reasons, a need exists for an image forming apparatus using direct recording methods without producing images having uneven image density.